Which routing protocol supports unequal cost, multipath load balancing?

RIPv2 supports only equal-cost load balancing across a maximum of four paths by default and has no variance or unequal-cost mechanism.

EIGRP's variance command allows the router to include feasible successor routes whose metric falls within a multiplier of the successor's metric, enabling load balancing across paths with different costs. Traffic is distributed proportionally based on each path's metric, so higher-metric paths carry proportionally less traffic. No other standard IGP (RIP, OSPF, IS-IS) supports unequal-cost load balancing natively.

OSPFv3 performs equal-cost multipath (ECMP) load balancing only and does not support unequal-cost path selection.

IS-IS also performs equal-cost multipath load balancing and lacks a built-in unequal-cost load-balancing feature comparable to EIGRP variance.